Hydrophone Experiments

I’ve been experimenting for several years with piezoelectric contact mics and hydrophones. Usually the results are disappointing. (Except for Alex Rice’s phantom-powered contact mic buffer circuit, which addresses many common piezo pitfalls.)

In 2009 I read a wonderful post on Mike Rooke’s Lab Blog which outlined several novel hydrophone designs. The one that really struck me was to seal a standard electret mic capsule in rubber. The advantage of this technique is a relatively flat frequency response (without the characteristic resonant “honk” of a piezo disc) and simple assembly. The thickness and composition of the silicone layer will affect the frequency response and sensitivity, so “simple” could get complicated if you want matched pairs. The night before a visit to the Bay of Fundy in August 2010, I built a “quick and dirty” pair and they exceeded my expectations.

Construction Notes

The housings are metal 1/4″ plugs with the tips snapped off, arranged “backwards”, with the mic capsule facing out where the springy metal strain-relief should be and the cable escaping out the hole left by the missing tip. (This arrangement is not ideal, but it adds enough mass to keep the hydrophones from floating.)

I built a jig to hold the capsule and then flooded the interior with silicone. After that dried, I slathered the exterior including a thin swipe over the mic capsule.

I used Panasonic WM-61a capsules, with the felt discs removed so the silicone could flow directly onto the diaphragm. This may be a mistake from an electrostatic perspective, but it appears to make a good impedance match between the water and the diaphragm. In the future I’ll try it both ways. Update 2014: I finally did a comparison between the method I describe here versus a stock capsule (where the felt disc keeps silicone from touching the diaphragm). The stock capsule had a much lower output, so I do recommend removing the felt disc.

This style of hydrophone probably won’t handle the pressures of deep water. Fun for splashing around though!